# [Gmsh] Medit .mesh problem

**Geordie McBain**
gdmcbain at freeshell.org

*Thu May 27 02:39:13 CEST 2010*

2010/5/26 Philippe Miron <philippe.miron at polymtl.ca>:
>* Hi everyone,
*>*
*>* I'm using the Medit .mesh file format to export my mesh generated with this
*>* test model (see attachment and generate the default 3D mesh). Question: is
*>* it normal that there are nodes numbers that doesn't appear anywhere in the
*>* Tetrahedra list (for exemple node # 1) ?
*>*
*>* Maybe I'm doing something wrong, but your help would really be appreciated!
*
Hello. There's not really a problem here. You haven't defined any
physical entities, so when Gmsh saves the mesh, it saves everything.
This means a vertex is saved for each geometric point, edges along
each geometric line, triangles over each surface, and tetrahedra
through each volume. Vertex 1 corresponds to Point (1), which is just
a construction point in your geometry, a centre of a circle. Actually
the meshing step ended up in your case putting a node right there at
the same coordinates (0, 0, 0), vertex 51, and that is mentioned in
Tetrahedra list.
Anyway, that's the long complicated answer. The short answer is that
you probably want to define physical entities for the pieces relevant
to your finite element problem. See
http://geuz.org/gmsh/doc/texinfo/gmsh.html#Elementary-vs-physical-entities.
Here you might want a Physical Volume for the cylinder and maybe two
or three Physical Surfaces for its ends and cylindrical wall. If you
do that, you probably also want to set Mesh.SaveElementTagType = 2, so
the MEDIT tags correspond to the Physical entities.